As the demand for hydrocarbon-based fuels has increased, the need for improved processes for separating hydrocarbon feedstocks of heavier molecular weight has increased as well as the need for increasing the conversion of the heavy portions of these feedstocks into more valuable, lighter fuel products. These heavier, “challenged” feedstocks include, but are not limited to, low API gravity, high viscosity crudes from such areas of the world as the Middle East, Mexico, Venezuela, Russia, as well as less conventional refinery feedstocks derived from such sources as bitumen, shale oil and tar sands. It is also important that heavy crude fractions, such as atmospheric resids, vacuum resids, and other similar intermediate feedstreams containing boiling point materials above about 850° F. are processed in such a manner so as to improve their ability to be utilized as feedstreams for subsequent refining and petrochemical processes such as, but not limited to, fuels blending, fuels upgrading, catalytic conversion, and lube oils production and upgrading.
Most conventional refineries have crude distillation units which are designed to distill lighter boiling point fractions from the heavier boiling point crude fractions. These distillations are typically performed in refineries in at least two discrete distillations. The first of these crude or heavy oil distillations is commonly termed as “crude atmospheric distillation” which is performed in a distillation tower with an overhead pressure regulated approximately at or slightly above atmospheric pressure (14.7 psia). The bottoms or heaviest fractions obtained from the atmospheric distillation process (generally termed as “atmospheric resid”) are sent for further distillation at sub-atmospheric distillation pressures. Generally, in a modern petrochemical refinery, the atmospheric resid is further distilled at these sub-atmospheric distillation pressures in what is generally termed as “crude vacuum distillation”. In some refineries, the atmospheric resid may undergo some intermediate distillation steps, but usually the heaviest portions of the atmospheric resid stream end up in the feedstream to the crude vacuum distillation process.
In the crude vacuum distillation process, the feedstream comprising atmospheric resid is further separated in sub-atmospheric distillation conditions. Typically, the crude vacuum distillation column is operated at below about 7 psia, but most preferably about 2 psia or lower. The decreased partial pressures of the feedstream allows heavier hydrocarbon fractions to be distilled in the crude distillation column than would be possible under atmospheric conditions at the same feed temperature thereby increasing the volume of intermediate boiling point products isolated in the crude distillation process.
Generally, the bottoms or heaviest fractions obtained from the crude vacuum distillation process (generally termed as “vacuum resid”) cannot be further separated by distillation and are sent for further processing in the refinery which either requires further separation of components by methods such as solvent extraction or thermal conversion. However, these processes can be very expensive to operate and/or can result in a significant amount of valuable component loss in the final product.
What is needed in the industry is a separations process which does not require extractive solvents and further separates the compounds in a heavy oil feedstream which does not thermally alter the overall composition of the final products. It is desired that the separated product streams from such a process have compositional qualities that will increase the value of at least one, but preferably all of product streams as intermediate product streams for further refinery/petrochemical processes or products.